Lean NO.sub.x traps operate cyclically. During the lean portion of the cycle (fill duration), NO.sub.x is adsorbed. After running lean for a period of time, the trapping efficiency becomes low and the trap must be regenerated. This is done by operating rich of stoichiometric. The hydrocarbons and CO emitted during rich operation causes the NO.sub.x to reduce to N.sub.2 and O.sub.2. The lean part of the cycle may typically last for one minute followed by the regeneration or purge part of the cycle for one second.
It is desirable that the transition between the fill and purge portions of the cycle be imperceptible to the driver. Accordingly, the prior art strategies provide for a relatively slow transition between the fill and purge portions of the cycle. Of the transitions that must occur, including fuel flow and spark changes, the most difficult transition is the manifold pressure change. The throttle (or other valve) must be closed rapidly to accomplish the lean to rich and rich to lean transition. Even after the throttle has attained the appropriate position, it takes a few cycles for the intake manifold to fill or to purge, depending on the transition. Thus, the transition occurs over a number of engine cycles. Because of the number of parameters which must be changed, it is a challenge to do this robustly and with no torque fluctuation. Furthermore, the ramping of the conditions which are required to get to and from the purge condition negatively impacts fuel economy.